Chapter 5: Cloud Development and Precipitation Atmospheric Stability Determining stability Cloud development and stability Precipitation processes Precipitation types Measuring precipitation

Atmospheric Stability

Atmospheric Stability stable and unstable equilibria air parcels adiabatic process adiabatic lapse rates Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

Determining Stability

A Stable Atmosphere environmental lapse rate absolute stability stabilizing processes subsidence inversions Stable air provides excellent conditions for high pollution levels.

An Unstable Atmosphere absolute instability warming of surface air destabilizing processes superadiabatic lapse rates Unstable air tends to be well-mixed.

Conditionally Unstable Air conditional instability dry and moist adiabatic lapse rates

Cloud Development and Stability

Cloud Development and Stability surface heating and free convection uplift along topography widespread ascent lifting along weather fronts

Convection and Clouds thermals fair weather cumulus Fair weather cumulus provide a visual marker of thermals. Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

Topography and Clouds orographic uplift rain shadow The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Precipitation Processes

Collision and Coalescence Process terminal velocity coalescence warm clouds A typical cloud droplet falls at a rate of 1 centimeter per second. At this rate it would take 46 hours to fall one mile.

Figure 5. 9: Conditionally unstable atmosphere Figure 5.9: Conditionally unstable atmosphere. The atmosphere is conditionally unstable when unsaturated, stable air is lifted to a level where it becomes saturated and warmer than the air surrounding it. If the atmosphere remains unstable, vertical developing cumulus clouds can build to great heights. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-9, p. 116

Ice Crystal Process cold clouds supercooled water droplets saturation vapor pressures over liquid water and ice accretion The upper portions of summer thunderstorms are cold clouds!

Figure 5. 22: The ice-crystal process Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 5-22, p. 124

Figure 5. 22: The ice-crystal process Figure 5.22: The ice-crystal process. The greater number of water vapor molecules around the liquid droplets causes water molecules to diffuse from the liquid drops toward the ice crystals. The ice crystals absorb the water vapor and grow larger, while the water droplets grow smaller. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-22, p. 124

Cloud Seeding and Precipitation silver iodide It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Precipitation in Clouds accretion ice crystal process

Precipitation Types

Rain rain drizzle virga shower Virga is much more commonly observed in the western US, because the humid climate of the eastern US reduces the visibility.

Snow snow fallstreaks dendrite blizzard Snowflake shape depends on both temperature and relative humidity.

Sleet and Freezing Rain rime Sleet makes a ‘tap tap’ sound when falling on glass.

Snow Grains and Snow Pellets graupel

Hail updraft cycles accretion A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

Figure 5.35: Hailstones begin as embryos (usually ice particles) that remain suspended in the cloud by violent updrafts. When the updrafts are tilted, the ice particles are swept horizontally through the cloud, producing the optimal trajectory for hailstone growth. Along their path, the ice particles collide with supercooled liquid droplets, which freeze on contact. The ice particles eventually grow large enough and heavy enough to fall toward the ground as hailstones. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Stepped Art Fig. 5-35, p. 134

Measuring Precipitation

Instruments standard rain gauge tipping bucket rain gauge It is difficult to capture rain in a bucket when the wind is blowing strongly.

Doppler Radar and Precipitation

Figure 5. 39: A microwave pulse sent out from the radar transmitter Figure 5.39: A microwave pulse sent out from the radar transmitter. The pulse strikes raindrops and a fraction of its energy is reflected back to the radar unit, where it is detected and displayed, as shown in Fig. 5.40. Stepped Art Fig. 5-39, p. 135